Adhesive surface mounting of devices to a supporting substrate is exemplified by the disclosure of U.S. Pat. No. 3,818,279 to Seeger, Jr. et al. It discloses use of a conductive elastomeric material containing electrically conductive particles for establishing random conductive paths between a substrate area and an overlying device. Such adhesives have no directionally conductive properties.
U.S. Pat. No. 4,339,785 discloses the mounting of a component to a printed circuit board by use of a structural adhesive.
U.S. Pat. No. 4,774,634 to Tate et al., discloses use of a structural adhesive connecting to the body of a device and used in combination with a flexible conductive adhesive at electrical contact areas. This combination produces a conductive connection between the leads of a circuit component and a supporting substrate, but the connection is again multidirectional.
The need to confine the area of conductivity when surface mounting a component by use of a conductive adhesive has led to development of anisotropically conductive adhesives, also known as Z-axis adhesives. Anisotropically conductive adhesives are a mixture of a nonconductive adhesive binder and conductive particles capable of forming electrically conductive paths between facing conductive surface areas when subjected to heat and pressure. By delineating the opposed areas at which pressure is exerted on such an adhesive, one can eliminate stray and unwanted conductive paths outside that area, where the conductive particles will be sufficiently separated from each other so that current will not flow through the composite mass. This eliminates the flow of current between adjacent conductive areas on the same substrate and between facing conductive surfaces separated by a distance greater than the minimal distance required to complete an electrical path through the adhesive mixture.
Surface mounting of components by use of an anisotropically conductive adhesive currently requires application of pressure during the course of setting or curing the adhesive. This typically requires usage of mechanical fixtures that must remain in place as the component and substrate are heated within an oven. Present use of such adhesives also requires redesign of the mechanical fixtures as any component features within a circuit are changed by subsequent developments.
The present invention has been developed to take advantage of the ability to apply conductive adhesive by screen printing or other types of printing technology. The use of printing technology to create electrical connections between components and a substrate is very rapid and economical. The inventive method also provides a high degree of flexibility for meeting changing circuit requirements, eliminating the necessity of redesigning mechanical fixtures to hold the circuit components in place as the conductive adhesive is being set.